1. Field of the Invention
The present invention relates to a method for forming an indium oxide film by way of electrodeposition, and a method for forming an indium oxide film by way of electroless deposition. The present invention also relates to a plating solution for use in the formation of an indium oxide film by way of electroless deposition. The present invention further relates to a substrate provided with said indium oxide film for a semiconductor element and a semiconductor element provided with said substrate. The semiconductor element includes a photovoltaic element (including a photoelectric conversion element), a self luminescent type element, a liquid crystal element, and the like.
2. Related Background Art
Indium oxide is very transparent and has an excellent electrically conductive property similar to tin oxide and zinc oxide. In this respect, indium oxide has been often used for industrial purposes. Particularly, it has been used in the production of semiconductor elements having a portion to which light is impinged and a voltage is impressed upon use. Besides, it has been also used as an antistatic coating material which can be applied to a transparent glass member or the like. In addition, indium oxide is superior to tin oxide with respect to the electric conductivity, and it is superior to zinc oxide with respect to the stability to environments, specifically, for instance, resistance to chemicals. In this connection, indium oxide has more opportunities to be used in comparison with tin oxide and zinc oxide. Particularly, in the case of an indium oxide incorporated with several percentages to some tens percentages of tin such as ITO (In2O3xe2x80x94SnO2), it is very transparent and has an extremely small electric resistance. Therefore, it has been widely used as a transparent and electrically conductive film in various semiconductor elements.
As for the method for the formation of an indium oxide film, there are known vacuum deposition film-forming methods such as chemical vapor deposition (CVD, evaporation, sputtering, and MBE. Besides, a wet-type film forming method by so-called sol-gel processing is also known. Of these, the sputtering film-forming method has been preferably used, because this film-forming method is advantageous in that a uniform indium oxide film can be easily formed at a relatively low temperature. For the sputtering film-forming method, there is known a reactive sputtering film-forming method in which a target comprising indium metal is sputtered in an atmosphere composed of O2 gas. This reactive sputtering film-forming method enables the formation of indium oxide film at a high speed.
However, any of these vacuum deposition film-forming methods has such disadvantages as will be described in the following. A specific film-forming apparatus provided with a vacuum vessel and an exhaust device is required. In the case where the film-forming apparatus is large-sized, the film-forming apparatus unavoidably becomes costly, resulting in a rise in production cost of an indium oxide film. In addition, in the case of forming a large area indium oxide film uniformly on a large area substrate, a gas introduction means including a control system for a gas introduced, a means for controlling the temperature of the substrate, a power supply equipment and the like must be specifically designed to conform the large area substrate, wherein the film-forming apparatus becomes more costly. Further, a certain period of time is unavoidably consumed for evacuating the inside of the film forming space and also for raising or lowering the substrate temperature. In the case of producing a semiconductor element, there are various restrictions in the process for the production thereof.
For the film-forming method by sol-gel processing, there are problems such that a temperature of several hundreds centigrade is required for sintering, and a number of pores are likely to be generated in the resulting indium oxide film. In addition, it is difficult to mass-produce a uniform indium oxide film product having a sufficiently low electric resistance.
Therefore, in view of providing particularly an optical semiconductor element such as a photovoltaic element (or a photoelectric conversion element) at a reasonable production cost, there is a demand for realizing an appropriate film-forming method which enables the effective formation of a large area transparent and electrically conductive indium oxide film having a uniform thickness and a uniform property on a large area substrate at a high speed and at a low substrate temperature without using large-sized apparatuses including evacuating and heating apparatuses as in the case of the vacuum deposition film forming method.
A principal object of the present invention is to provide a method which enables the effective formation a high quality indium oxide film by way of electrodeposition or electroless deposition, which meets the foregoing demand and which can be employed in the production of various semiconductor elements.
Another object of the present invention is to provide a method for forming an indium oxide film, comprising the steps of: immersing an electrically conductive substrate as an electrode and a counter electrode in an aqueous solution containing at least nitrate ion and indium ion and applying an electric current between said substrate and said counter electrode, thereby causing the formation of said indium oxide film on the substrate.
A further object of the present invention is to provide a substrate for a semiconductor element comprising an electrically conductive substrate having an indium oxide film formed thereon by immersing said electrically conductive substrate as an electrode and a counter electrode in an aqueous solution containing at least nitrate ion and indium ion and applying an electric current between said substrate and said counter electrode.
A further object of the present invention is to provide a photovoltaic element comprising at least a first transparent and electrically conductive layer, a semiconductor layer and a second transparent and electrically conductive layer stacked in this order on an electrically conductive substrate, wherein at least one of the first and second transparent and electrically conductive layers comprises an indium oxide film formed by immersing said electrically conductive substrate as an electrode and a counter electrode in an aqueous solution containing at least nitrate ion and indium ion and applying an electric current between said substrate and said counter electrode.
A further object of the present invention is to provide an aqueous solution containing at least nitrate ion, indium ion and tartrate for use in the formation of an indium oxide film by way of electroless deposition.
A further object of the present invention is to provide an electroless deposition film-forming method for forming an indium oxide film, comprising the steps of: immersing a substrate in an aqueous solution containing at least nitrate ion, indium ion and tartrate and maintaining said substrate at a temperature of 10 to 60xc2x0 C., thereby depositing said indium oxide film on said substrate.
A further object of the present invention is to provide a photovoltaic element comprising at least a first transparent and electrically conductive layer, a semiconductor layer and a second transparent and electrically conductive layer stacked in this order on a substrate, wherein at least one of the first and second transparent and electrically conductive layers comprises an indium oxide film formed by immersing said substrate in an aqueous solution containing at least nitrate ion, indium ion and tartrate and maintaining said substrate at a temperature of 10 to 60xc2x0 C. to deposit said indium oxide film on said substrate.